GFS2: Fix kbuild test robot reported warning

Well I don't get the same warning locally as the kbuild
robot, but I guess this should fix the problem, anyway.
Here is the warning:

head:   2d9e7230
commit: ee2411a8 [19/20] GFS2: Clean up quota slot allocation
config: make ARCH=powerpc allmodconfig

All error/warnings:

   fs/gfs2/quota.c: In function 'gfs2_quota_init':
>> fs/gfs2/quota.c:1246:3: error: implicit declaration of function '__vmalloc' [-Werror=implicit-function-declaration]
      sdp->sd_quota_bitmap = __vmalloc(bm_size, GFP_NOFS, PAGE_KERNEL);
      ^
>> fs/gfs2/quota.c:1246:24: warning: assignment makes pointer from integer without a cast [enabled by default]
      sdp->sd_quota_bitmap = __vmalloc(bm_size, GFP_NOFS, PAGE_KERNEL);
                           ^
   fs/gfs2/quota.c: In function 'gfs2_quota_cleanup':
>> fs/gfs2/quota.c:1361:4: error: implicit declaration of function 'vfree' [-Werror=implicit-function-declaration]
       vfree(sdp->sd_quota_bitmap);

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>